1. Field of the Invention
The present invention relates to a wire derailment detecting apparatus of a wire saw, and more particularly to a wire derailment detecting apparatus of a wire saw which slices a workpiece such as a semiconductor ingot into a number of wafers by a running wire.
2. Description of the Related Art
In a wire saw, a wire, which runs at a high speed between a pair of wire reels, is wound on a plurality of grooved rollers to form a wire row. The workpiece is pressed against the wire row supplied with manufacturing solution including abrasive grains. As a result, the workpiece can be sliced into a number of wafers by lapping action of the abrasive grains.
In the wire saw, the wire must be prevented from snapping while the workpiece is being sliced. The wire is snapped mainly because the wire is derailed from the grooved rollers forming the wire row, and because the wire is derailed from the guide roller guiding the running wire.
In order to detect whether the wire is derailed, there is a method in that a pair of the wire reels are electrically insulated from a body of the wire saw, and a constant voltage is applied between the wire reels. Then, when a drop in the voltage is detected, then it is determined that the wire is derailed. This method is based on the following facts. That is, many of the grooved rollers and the guide rollers are made of resin such as rubber, polyurethane, polyethylene, or the like. If the wire is normally wound on the rollers, there is a constant difference in voltage between the wire reels. If the wire is derailed, the wire contacts the body of the wire saw, and the voltage is dropped.
The current manufacturing solution of the wire saw is usually oil-based. The oil-based manufacturing solution is faced with problems in which the sliced wafer cannot be cleansed sufficiently, the user becomes stained with the solution, and the environment is polluted. Therefore, a water-based manufacturing solution has been recently developed to an extent that it can be put to practical use.
However, if the conventional method of electrically detecting the derailment is applied to the wire saw performing the slicing by means of the water-based manufacturing solution, the manufacturing solution conducts electricity. Thus, there are disadvantages in that the operation is incorrect and the detection is delayed.